Probing discontinuous precipitation in U-Nb

Abstract

U-Nb's discontinuous precipitation, $\gamma^{bcc}_{matrix} \rightarrow \alpha^{orth}_{cellular} + \gamma'^{bcc}_{cellular}$, is intriguing in the sense that it allows formation and growth of the metastable $\gamma'$ phase during the course of its occurrence. Previous attempts to explain the thermodynamic origin of U-Nb's discontinuous precipitation hypothesized that the energy of $\alpha$ forms an intermediate common tangent with the first potential of the double-well energy of $\gamma$ at the $\gamma'$ composition. While this hypothesis is eligible and consistent with the experimental observation of gradual increase in $\gamma'$ composition at increasing temperature, it is in conflict with recent experiments whose results indicated a distribution of $\gamma'$ compositions in the vicinity of 50 at\%Nb. To shed some light onto this issue, the current work investigates the origin of U-Nb's discontinuous precipitation in view of fundamental thermodynamics and kinetics, taken from the perspective of phase-field theory. It has been showed that local misfit strain tends to play a crucial role in the formation and growth the discontinuous precipitation. Depending on the magnitude of strain developed at grain boundaries, either an increasing $\gamma'$ composition or a random distribution of $\gamma'$ composition around the equiatomic value with respect to increasing temperature could be expected.